The present invention relates to a wood-polymer composite, particularly wood which has been impregnated with a mixture containing polymerizable organic compounds, and more particularly to a method for preparing a wood-polymer-composite by impregnating wood material and/or wood-based material with a mixture containing polymerizable organic compounds, and most particularly uses thereof.
Wood-polymer composites of this category is based on polymerization of a monomer in the cavities of the cells (in-situ). The polymer produced is usually a thermoplastic (linear polymer). Monomers which belong to the vinyl monomer group are the ones usually used for this purpose. The vinyl monomers used are polymerized by means of radical chain polymerization. The vinyl monomers are nonpolar, and will therefore not swell or react with the cell wall of the wood material. Some persons skilled in the art therefore claim that wood-polymer materials of this type are not  less than  less than real greater than  greater than  composites because there is no chemical bonding between the two combined materials. However, there is a high extent of infiltration and the material acquires altered physical and mechanical properties, and hence should be defined as a novel type of material. Properties which are substantially increased are density, hardness, wear resistance and elastic modulus. Liquid water and water vapor movements in the new material are greatly reduced, especially along the grain. In wood, transport along the grain is many times greater than across the grain, but in the new material they are about equal. Because the polymer does not react with the structure of the cell walls, the basic properties of the woody cell wall are not changed. Because the pores of the wood are blocked by polymer, the moisure can only move within the unchanged woody cell wall, in spaces between the cell wall and the polymer in the lumen, or in any cracks that might be in the material. The time for humidification or drying of cell lumen wood-polymer composites will therefore be 10 to 20 times longer than for untreated wood. This delay is an advantage in changing environments (especially a spill of liquid water). Because of the longer time, and because the polymer may induce a mechanical restraint of swelling, the dimensional stability of the new material is slightly improved. However, this can lead to local stresses in the material, which can result in microcracks when the material is subjected to extreme moisture gradients.
The monomers usually utilized for cell lumen wood-polymer composite are methyl methacrylate and styrene and unsaturated polyester oligomers, because of large supply of these chemicals and hence their low cost, as well as the relatively simple method of polymerizing them. Negative aspects of methyl methacrylate monomer are its subtantial shrinkage (up to 25%) during polymerization and relatively high vapor pressure (it evaporates easily). Negative aspects of styrene monomer are its high vapor pressure (it evaporates easily) and easily-detectable odour (low odour threshold). A negative aspect of polyester oligomer is its higher viscosity which limits impregnation possibilities.
Monomer evaporation can lead to a low filling of surfaces by monomer. Monomer shrinkage can lead to substantail shrinkage and warping of materials during cure, and can cause voids between polymer and cell wall. By selecting different types of monomers these problems may be reduced.
One problem of previously known wood-polymer composites of this category is that after impregnation and curing, they continue to liberate vinyl monomers which have an easily-detected odour and which can be irritating and injurious to health.
Other problems of previously known wood-polymer composites are cracking, warping and colour alteration of the materials. These indicate that temperatures of the curing phase caused by exothermic heat of reaction have been too high, resulting in excess shrinkage and pyrolysis damage to the wood. The problem is increased dramatically with increased cross-sectional size, and in the past has limited the size of material which could be successfully treated.
Another problem of previously known wood-polymer composites results from a combination of moisture in the wood and the curing heat. Because of the exothermic heat of reaction, the temperature, especially of larger cross-sections, exceeds the boiling point of water. This causes rapid drying, followed by warping and cracking in moist wood. For this reason, previous art successful treatment has been generally limited to dry (6% moisture content and less) wood.
It is the object of the invention to provide a polymer impregnated wood of higher moisture content and having improved hardness, which also has low odour, and further, little cracking is observed in the finished material, thus eliminating the above disadvantages.
Essentially, this object is accomplished by using a wood material as a starting material having a moisture content from about 3% to about 90%, in particular from about 15% to about 35%, especially from about 20% to about 30%, and preferably about 25%, and subjecting said starting material to specific polymerizable monomer formulations and impregnation and curing conditions.
The specific mixture of polymerizable monomers and additives combined with a specific curing temperature schedule is important for producing odourless materials.
When wood-polymer composites are manufactured, moisture may be a problem, because the reaction exotherm causes temperatures above the boiling point of water in larger cross sections regardless of the heating method used. Rapid dehydration results. This causes deformations and cracks.
An essential feature of the present invention is to use starting wood materials having high moisture content. It is not prior knowledge to produce wood-polymer composites using moist wood as the starting material.
Wood, including cheap types and scrap material, can be used to produce noble wood materials such as imitation teak, mahogany, and others, and also provide them with novel properties like water resistance and simpler and reduced maintenance requirements. Since the monomers used are colourless, colours can be added, and thereby the finished product can be coloured throughout
According to the present invention, the foregoing and other objects are attained by a product, method and uses thereof as disclosed in the patent claims.
An embodiment of this invention comprises a wood-polymer composite, characterized by wood impregnated with polymerizable monomers selected from a group consisting of styrene, methylstyrene and tertiary butylstyrene, initiated with at least three initiators, crosslinked with divinyl benzene, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, ethylene glycol trimethacrylate or trimethylol propane trimethacrylate and containing oil or wax as a polymerization moderator and water repellent for the finished product.
A second embodiment of this invention comprises a method for preparing a wood-polymer composite by impregnating wood material and/or wood-based material, characterized by the steps of:
utilizing said wood material having a moisture content of from about 15 to about 35% based on the weight of said wood material; impregnating said wood material with a composition selected from a group consisting of polymerizable monomers, a crosslinker and at least three initiators by means of vacuum and/or pressure; and curing said impregnated wood material in a hot air oven, steam or hot oil by means of a temperature sufficient to achieve the desired polymer loading.
A third embodiment of this invention comprises use of wood-polymer composite as a doorstep, doorlaminate, floor, handle, building board, pallet, acoustic wood, outdoor furniture, indoor furniture, container floor, play apparatus, benchtop, outdoor deck material, stair and railing material, fence stakes, or timber.
Particularly, said timber is a railway sleeper, especially an environmentally-friendly railway sleeper. Also, said timber can be a pole, especially an environmentally-friendly pole.
However, the use of said wood-polymer composite should not be restricted to those indicated above, but can include generally all kinds of use of wood products.
To obtain complete cure and low emissions from the finished product, it was found that a combination of initiators is needed. A lower temperature initiator starts the reaction, a second (higher temperature) carries it on and a third (highest temperature) finishes it. In most of these categories, alternative initiators may be used, depending on the reaction conditions, plant safety, cost and end-use requirements. Oil or wax is used to reduce the reaction exotherm during polymerization for the larger sizes of material, and also to act as water repellant for the material. The oil or wax has an additional benefit in that it is lower cost than monomer, reducing cost of the treatment.
The main monomers (styrene, methylstyrene (particularly para-methylstyrene) and tertiary butylstyrene) can be used alone or in any combination. The main advantage of methylstyrene and tertiary butylstyrene is their lower vapor pressure and thus lower emissions in the production plant and the finished product.
Quantities of the initiators and crosslinkers used depend on product, process and end-use but are between a minimum and maximum value for each. Below the minimum value it will not work; above the maximum value and the formulation will work but it is unnecessary and costly. Quantity of oil or wax is dependent on product size, curing conditions and desired end-use properties.
The mixtures used are shown in Table 1.
The following Examples will further illustrate the invention.